linux-kernel - Re: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch

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Date:   Thu, 30 Aug 2018 16:17:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
Cc:     "will.deacon@....com" <will.deacon@....com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Alexey.Brodkin@...opsys.com" <Alexey.Brodkin@...opsys.com>,
        "Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "yamada.masahiro@...ionext.com" <yamada.masahiro@...ionext.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch_"
 causes kernel crash

On Thu, Aug 30, 2018 at 11:53:17AM +0000, Eugeniy Paltsev wrote:
> I can see crashes with LLSC enabled in both SMP running on 4 cores
> and SMP running on 1 core.

So you're running on LL/SC enabled hardware; that would make Will's
patch irrelevant (although still a good idea for the hardware that does
care about that spinlocked atomic crud).

Does something like the below cure things? That would confirm the
suggestion that the change to __clear_bit_unlock() is the curprit.

If that doesn't cure things, then we've been looking in entirely the
wrong place.

---
diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h
index 3ae021368f48..79c6978152f8 100644
--- a/include/asm-generic/bitops/lock.h
+++ b/include/asm-generic/bitops/lock.h
@@ -57,12 +57,7 @@ static inline void clear_bit_unlock(unsigned int nr, volatile unsigned long *p)
 static inline void __clear_bit_unlock(unsigned int nr,
 				      volatile unsigned long *p)
 {
-	unsigned long old;
-
-	p += BIT_WORD(nr);
-	old = READ_ONCE(*p);
-	old &= ~BIT_MASK(nr);
-	atomic_long_set_release((atomic_long_t *)p, old);
+	clear_bit_unlock(nr, p);
 }
 
 /**